This invention relates to a method for synchronizing the clock signal in a digital data receiver to decode pulse position encoded signals.
In pulse position modulation (PPM) the time position of pulses carries the message to be communicated. The positioning of pulses is referenced to a clock signal which defines slot times in which signals are expected to appear. For example, at a clock frequency of 14.7 MHz, and using ten clock signals to define a slot, each slot time is 680 nanoseconds long. For a 32-slot PPM frame, a pulse appearing in the first slot would represent the number 0, a pulse appearing in the second slot would represent the number 1, and so on, counting upward to the last slot which would be the number 32, or in hexadecimal representation #1 F. Thus, we can see that five binary bits are encoded and transmitted in a 32-slot PPM frame.
When receiving a PPM signal, it is necessary to decide in which slot an arriving pulse belongs, usually in "real time" as the pulses are received. If a pulse is incorrectly assigned to the wrong slot, due to variations in the time position of the pulse or the slot, the digital signal is incorrectly decoded. It is desirable to have a method for correctly assigning pulses to slots, and for correcting variations in the time positions of the slots by changing the phase or timing of the clock signal.